In a mobile receiving environment where a wave, in which a reflected wave, a diffracted wave, a scattered wave and the like are combined, arrives at a receiving point through various propagation paths, multipath fading where a receiving power level wildly fluctuates is generated in the radio signal demodulating device. The quality of a received signal deteriorates in the radio signal demodulating device due to this multipath fading. Therefore, in the radio signal demodulating device used in the mobile receiving environment, to improve the quality of the received signal is an essential technical problem. Specifically, it is necessary to develop a radio signal demodulating device suppressing interference from multipath fading and improving a Desire power to Un-desire power Ratio (DUR) of a demodulated signal.
A diversity technology with multiple antennas which can receive a desired signal effectively is widely practiced as a technology of reducing the influence of the multipath fading. In the diversity technologies, a switching diversity, which switches antenna output in accordance with a receiving power of each antenna, has a simple configuration so that it is most frequently used as a radio signal demodulating device in the mobile receiving environment.
As another diversity technology, an adaptive array antenna system, which independently controls the amplitude and phase of a signal received in each antenna element by a signal processing so as to adaptively perform the control of directional characteristics of the entire system, is under consideration. This system has characteristics of maximizing DUR of a desired signal as well as suppressing spurious waves, the characteristics depending on an algorithm for calculating a weight factor used for combining a signal.
In the mobile receiving environment, it is difficult to accurately recognize information regarding the arriving direction and arriving wave number of a received signal, so that the Minimum Mean Square Error (MMSE) method not requiring the aforementioned information is often used for this algorithm
Furthermore, the algorithms of this MMSE contains a Constant Modulus Algorithm (CMA) as an algorithm of maximizing the DUR on the premise that a sending signal is a constant envelope signal such as a Frequency Modulation (FM) wave, a Phase Modulation (PM) wave and the like. This CMA is an algorithm on the basis of the principle that an envelope is constant.
The most distinctive feature of this CMA is that it is a blind algorithm not requiring replica of a desired signal. Thus, this CMA with a simple configuration is applicable to a radio signal demodulating device, and has been already put in use. It should be noted that the aforementioned switching diversity and the radio signal demodulating device using the CMA is described in detail in Patent Document 1 and Patent Document 2.
Also, the adaptive array antenna system is disclosed in Patent Document 3. This Patent Document 3 discloses an adaptive array antenna system compensating distortion of the phase and amplitude of an output signal by using pilot signals which are time-division-multiplexed to a data signal, to produce a reference signal and having the effect of preferably receiving a desired signal.
Patent Document 1: Japanese Patent Application Laid-Open No. H07-336130 (1995)
Patent Document 2: Japanese Patent Application Laid-Open No. 2005-217849
Patent Document 3: Japanese Patent Application Laid-Open No. H07-154129 (1995)
However, in the radio signal demodulating device using the conventional switching diversity, the phase of a signal is turned to be discontinuous along with the switching of an antenna element, thereby causing a problem of deteriorating a receiving performance. Further, in the radio signal demodulating device using the conventional switching diversity, there has been also a problem that even in a transmission channel environment where spurious waves are present, the spurious waves are combined without being suppressed.
The radio signal demodulating device using the conventional CMA takes time to converge to an optimum receiving state, and has a limit in convergence performance for the fluctuation of the transmission channel environment under the mobile receiving environment. Further, in the radio signal demodulating device using the conventional CMA, there has been a problem of receiving spurious waves when the power of the spurious waves is above the power of a signal desired to receive under the transmission channel environment where the spurious waves are present.
Also, the radio signal demodulating device employing the configuration of Patent Document 3 has a problem that the distortion of the phase and amplitude of the output signal are not compensated simultaneously when a data signal is not a complex signal (for example, a data signal is a signal obtained by demodulating a frequency-modulated signal).